Lucy spacecraft to fly by asteroid Donaldjohanson this Easter
April 16, 2025
Lucy spacecraft to fly by asteroid Donaldjohanson this Easter
Asteroid (52246) Donaldjohanson from a distance of 960 kilometres
During its second encounter with an asteroid, NASA's Lucy spacecraft captured a close-up view of asteroid Donaldjohanson on 20 April 2025. Measuring eight kilometres in length, the asteroid is larger than expected. This image is one of the highest-resolution images taken by the L'LORRI camera onboard Lucy, revealing landscape features down to 50 metres in size. The different densities of impact craters on the two parts of the binary system are striking.
Image: 1/7, Credit:
NASA/Goddard/SwRI/Johns Hopkins APL
Asteroid flyby for NASA's Lucy mission
An artist's impression of NASA's Lucy spacecraft approaching the asteroid (52246) Donaldjohanson, which measures nearly four kilometres across. Lucy’s flyby on 20 April 2025, at a distance of 960 kilometres and a speed of 13.4 kilometres per second, will serve as a dress rehearsal for the study of the trojan asteroids in Jupiter's orbit, which the probe will reach in 2027/28 and again in 2033. The science team expects the satellite images of Donaldjohanson to reveal features as small as 50 metres.
At distances of 700 to 800 million kilometres from the Sun – Jupiter's orbit, like the orbits of all planets, is not perfectly circular but elliptical – the power density of solar radiation averages only 12.6 watts per square metre. By comparison, Earth receives 1361 watts per square metre, approximately 100 times more solar energy. In order for a spacecraft like Lucy to still be able to operate using solar power at these distances from the Sun (a technically impossible feat two decades ago), it needs two very large solar panels, each 7.3 metres in diameter. At the mission's furthest distance from the Sun, the panels can supply the spacecraft with 504 watts of power.
On 20 April 2025, NASA's Lucy spacecraft will fly past asteroid (52246) Donaldjohanson, which measures nearly four kilometres in diameter, at a distance of 960 kilometres.
This serves as a dress rehearsal for Lucy's exploration of the trojan asteroids in orbit around Jupiter, which the mission is expected to reach in 2027/2028 and again in 2033.
The science team is expecting surprises from this C-type asteroid.
The asteroid is named after the paleoanthropologist Donald Johanson, who discovered the fossilised partial skeleton of the hominin named 'Lucy'.
Focus: Space, exploration, asteroids
+++ Update: NASA's Lucy spacecraft has flown past asteroid (52246) Donaldjohanson at a distance of 960 kilometres, sending back its first razor-sharp images to Earth. Transmitting all recorded data will take about a week. Donaldjohanson is larger than expected, measuring approximately eight kilometres in length and 3.5 kilometres across at its widest point. The asteroid is shaped like a dumbbell, with two differently sized 'weights' connected by a narrow, cylindrical 'handle'.
The asteroid is a contact binary, consisting of two asteroids that have merged. The larger of the two bodies appears older, as it has significantly more impact craters than the smaller part of the binary system. Donaldjohanson's geology seems to be complex.
Stefano Mottola from the DLR Institute of Planetary Research, a member of the Lucy science team and currently working at the Southwest Research Institute in Boulder, Colorado (USA), explains: "As we approached Donaldjohanson, we observed large fluctuations in brightness over a period of ten days. This led us to suspect that we were dealing with an elongated binary body. However, the unusual shape of the narrow neck still came as a surprise to our team." +++
Animation of individual images from Lucy's flyby of Donaldjohanson
This time-lapse animation strings together a series of individual images of asteroid Donaldjohanson, as seen by the Lucy spacecraft during its close flyby. The asteroid has a smooth, light-grey surface with soft craters pitting its surface. The larger of the two lobes in this binary system shows many more craters. The individual images were taken on 20 April 2025, starting at 19:50 CEST, from distances ranging between 1600 to 1100 kilometres, at intervals of approximately two seconds. The asteroid itself rotates very slowly; the apparent rotation in this animation is due to the movement of the spacecraft.
Credit:
NASA/Goddard/SwRI/Johns Hopkins APL
On Easter Sunday, at 19:51 CEST, NASA's Lucy spacecraft will perform a close flyby of the asteroid (52246) Donaldjohanson, at a distance of just under 1000 kilometres. The flyby distance has been precisely calculated to allow the L'LORRI camera system to capture sharp, blur-free images despite the spacecraft's high relative speed. This will enable detailed images to be taken with a resolution of up to ten metres per pixel, capable of showing features as small as 50 metres. Donaldjohanson is approximately four kilometres in diameter and resides within the main asteroid belt between Mars and Jupiter. The German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) is involved in the Lucy mission and will begin analysing images and data immediately following the flyby.
A high degree of precision is required for the flyby, which will take place approximately 225 million kilometres from Earth. "Lucy will shoot past its target at 13.4 kilometres per second," explains Stefano Mottola from the DLR Institute of Planetary Research and a member of the Lucy science team. "That's nearly 50,000 kilometres per hour. This is why we observed Donaldjohanson from a great distance using the camera system over a 39-day period in February and March. On the one hand, this allowed us to better characterise the asteroid; on the other, we were able to refine the spacecraft's trajectory if necessary. That turned out not to be needed – Lucy remained precisely on course." The radio signal from Lucy will take 12.5 minutes to reach Earth.
This existing sequence of optical navigation images illustrates how Donaldjohanson brightens as the spacecraft draws nearer. In a kind of time-lapse flipbook, successive images – each with a field of view of 0.3 degrees – are superimposed to show Lucy's approach. At the start of the sequence, Lucy was still more than 72 million kilometres away from the asteroid; by the end, the distance had closed to just 25 million kilometres.
This series of optical navigation images, acquired by the L'LORRI camera system on NASA's Lucy spacecraft, shows asteroid Donaldjohanson gradually brightening over 39 days in February and March 2025 as the spacecraft approaches the target asteroid. Lucy will pass Donaldjohanson on 20 April 2025, at a distance of 960 kilometres. In this time-lapse sequence, consecutive optical navigation images are superimposed with a field of view of 0.3 degrees. Lucy begins at a distance of over 72 million kilometres from the asteroid and ends only 25 million kilometres away. The asteroid appears to move in an arc relative to the background stars as the spacecraft – which was initially behind Donaldjohanson in its orbit around the Sun – catches up with and temporarily overtakes the asteroid.
Lucy's approach to asteroid Donaldjohanson
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Lucy's approach to asteroid Donaldjohanson
This series of optical navigation images, acquired by the L'LORRI camera system on NASA's Lucy spacecraft, shows asteroid Donaldjohanson gradually brightening over 39 days in February and March 2025 as the spacecraft approaches the target asteroid. Lucy will pass Donaldjohanson on 20 April 2025, at a distance of 960 kilometres. In this time-lapse sequence, consecutive optical navigation images are superimposed with a field of view of 0.3 degrees. Lucy begins at a distance of over 72 million kilometres from the asteroid and ends only 25 million kilometres away. The asteroid appears to move in an arc relative to the background stars as the spacecraft – which was initially behind Donaldjohanson in its orbit around the Sun – catches up with and temporarily overtakes the asteroid.
Credit:
NASA/Goddard/SwRI/Johns Hopkins APL
Automatic camera tracking for optimal image capture
To make optimal use of the L'LORRI (Lucy Long Range Reconnaissance Imager) camera system’s image sensor at the planned distance – meaning to keep as much of the asteroid's surface in the centre of each frame as possible – the spacecraft is equipped with a Terminal Tracking System (TTS). By automatically adjusting the camera's alignment to maintain focus on the target, the TTS locks on to the asteroid during the approach. "This worked extremely well during the first asteroid flyby in November 2023," says Mottola, confident that the system will perform just as reliably during Lucy's second close asteroid encounter. "The TTS enables us to make full use of the camera's field of view when imaging the target." In addition to this second test of both the TTS and the L'LORRI, which was developed by the Applied Physics Laboratory at Johns Hopkins University in Maryland, USA, the spacecraft's four other scientific instruments will also be activated to take measurements. The DLR Institute of Planetary Research will use the image data to generate digital terrain models, allowing scientists to determine the asteroid's shape and map its topography. The Institute will also contribute to the scientific interpretation of the images.
In Lucy's navigation images taken in February and March, the spacecraft was also able to detect periodic fluctuations in Donaldjohanson's brightness, caused by the asteroid's rotation, superimposed on the overall brightening as the asteroid draws nearer. The brightness fluctuations suggest that Donaldjohanson is elongated and rotates slowly, but this hypothesis can only be confirmed following detailed analysis of the images captured during the encounter on 20 April. During the spacecraft's long approach, Donaldjohanson will remain an unresolved point of light. The first visible surface details will only emerge on the day of the flyby.
Will the science team be in for another surprise?
In November 2023, Lucy successfully flew by the small main-belt asteroid Dinkinesh, which measures just 800 metres across, and discovered a 220-metre moon orbiting it, later named Selam. But the surprises did not end there. After the encounter, Lucy looked back and captured images revealing that Selam is in contact with its own small moon, making it a contact binary – two objects stuck together as one. If Donaldjohanson were also revealed to be a binary asteroid, like Dinkinesh, it would be a significant and unexpected discovery that would support the growing hypothesis that binary systems are far more common in the main asteroid belt than previously assumed.
Asteroid Donaldjohanson was discovered in 1981 by American astronomer Schelte John Bus at the Siding Spring Observatory in Australia. It is named after anthropologist Donald Johanson, who famously discovered the fossilised skeleton of a human ancestor known as 'Lucy'.
NASA's Lucy mission is itself named after this landmark find. Donald Johanson – born in 1943 – led a 1974 expedition to Ethiopia’s Afar Triangle near the Red Sea. There, he and his students unearthed the 3.2-million-year-old remains of an early 'pre-human', later classified as Australopithecus afarensis in the field of anthropology. The Donaldjohanson asteroid orbits the Sun approximately every three years, at a distance ranging from 291 to 420 million kilometres. Its orbit is tilted at 4.3 degrees relative to the ecliptic – the plane of Earth's orbit around the Sun.
Lucy will reach Jupiter's asteroids in 2027
Lucy is one of NASA's Discovery-class missions, launched on 16 October 2021. Its primary targets are the trojan asteroids located in Jupiter's orbit, which circle the Sun at the gravitationally stable L4 and L5 Lagrange points – 60 degrees in front of and behind the giant planet. Relatively little is known about these two groups of asteroids. They may differ in composition from the main belt asteroids, and some scientists speculate they have a different origin. It is possible they formed beyond the orbits of Jupiter and Saturn, and may even originate from the Kuiper Belt – a distant, doughnut-shaped region of icy bodies located between five and ten billion kilometres from the Sun. Lucy will observe six of these objects in Jupiter's orbit between 2027 and 28, and two more in 2033. After twelve years, Lucy will have closely observed a total of eleven asteroids.
